Semiconductor memory today is based on binary systems. In each cell of a binary system memory, only two states can be stored; namely, "1" or "0". If the number of states in a memory cell can be increased without increasing the area of the cell, then the memory density can be increased. Since semiconductor memory constitutes the most widely used product of integrated circuits today, increasing the memory density is highly desirable.
In the co-pending and allowed U.S. Pat. application Ser. No. 07/590,139, a multi-valued memory cell was disclosed utilizing the folding voltage-current characteristics. However, the disclosed circuit, as well as the circuit disclosed by Matsukawa et al in U.S. Pat. No. 4,573,143, used only one polarity for power supply and the device folding V-I characteristic in the third quadrant was not utilized to increase the number of memory states.